An insulated wire may be used in equipment such as heating equipment or a fire alarm, which requires a safe operation even under high temperature operating conditions. Further, insulated wires are also used in an automobile, wherein the vicinity of the engine, for example, is heated to a high temperature. An insulated wire formed as a conductor which is coated with a heat resistant organic resin such as polyimide, fluorocarbon resin or the like, has generally been used for the above purposes.
For applications requiring a nigh heat resistance or use of the wire in an environment requiring a high degree of vacuum, an organic coating has an insufficient heat resistance and does not satisfy the no gas emission requirement. Thus, an insulated wire of such a form that a conductor is inserted in an insulator tube of ceramics, an MI cable (Mineral Insulated cable) of such a form that a conductor is inserted in a heat resistant alloy tube of a stainless steel alloy etc. which is filled with metal oxide powder of magnesium oxide etc., or the like has been used under the above operating condition.
A fiber-glass braided insulated wire employing textile glass fiber as an insulating member is listed as an insulated wire having a certain flexibility and heat resistance.
The aforementioned insulated wire coated with an organic resin has such a heat resistance that the highest temperature at which insulability can be maintained, is about 200.degree. C. at the most. Therefore, it has been impossible to employ such an organic insulation coated wire under conditions which require a guarantee of the insulability under a high temperature of at least 200.degree. C.
Further, the insulated wire which is improved in its heat resistance by an insulator tube of ceramics, has the disadvantages of an inferior flexibility. An MI cable is formed by a heat resistant alloy tube and a conductor in the tube, whereby the outer diameter of the cable is increased relative to the conductor radius. Thus, the MI cable has a relatively large cross-section with respect to electric energy allowed by the conductor encased in the heat resistant alloy tube. In order to use the MI cable as a wire for winding a coil on a bobbin or the like, however, it is necessary no bend the heat resistant alloy tube along a prescribed curvature. Performing the bending of the heat resistant alloy tube is difficult. When winding the MI cable into a coil it is also difficult to improve the space factor since the tube forming the outer cable layer is thick as compared with the conductor.
Further, when the fiber-glass braided insulated wire having a certain heat resistance is employed and worked into a prescribed configuration in accordance with its use, the network of the braid is disturbed to cause a breakdown. In addition, dust of glass is generated by the glass fibers. This glass dust may serve as a gas adsorption source. Therefore, when the fiber-glass braided insulated wire is used in an environment requiring a high degree of vacuum, it has been impossible to maintain a high degree of vacuum due to the gas adsorption source provided by the glass dust.
A so-called alumite wire is manufactured by performing an anodic oxidation treatment on a wire of aluminum or an aluminum alloy to provide an insulated wire which has an excellent heat resistance, insulability and heat dissipation ability. However; the base material of the alumite wire is restricted to aluminum. Further, an inorganic insulating layer formed on the base material of the aluminum wire is also restricted to aluminum oxide. Thus, there has been a problem in that it is impossible to select combinations of the base conductor wire material and the inorganic insulating layer which are suitable for various uses.
U.S. Pat. No. 3,109,053 (Ahearn), issued Oct. 29, 1963, discloses an insulated conductor wire with a core of silver or copper, an intermediate layer of chromium, rhenium, iron or alloys thereof electroplated onto the core, and a vitreous insulating coating on the intermediate layer. The surface of the intermediate layer is oxidized after completion of the plating. Three different oxidizing procedures are disclosed by Ahearn. First, the plated wire core is heated to 600.degree. C. to 800.degree. C. in air or oxygen. Second, the plated wire core is anodized in an electrolytic bath. Third, the plated surface is coated with a ceramic composition containing an oxidizing agent and heating briefly to the curing temperature of the ceramic. These oxidizing procedures especially heating the plated chromium under the influence of air or oxygen has a strong tendency to form chromic oxide Cr.sub.2 O.sub.3 in which the atomic ratio of oxygen to Cr is 3/2=1.5 and chromic oxide does not provide an optimal two-fold adhesion first between the conductor core and the chromic oxide layer and second between the insulating coating and the chromic oxide layer. Tests set forth below show this point. In fact, Ahearn states that in order to further promote adherence of the vitreous insulating coating to the chromic oxide layer the latter's surface must be toughened by sandblasting, acid etching anti the like. This poses a further problem, because the intermediate layer should be as thin as possible and sandblasting may remove the intermediate layer.